1. Field of the Invention
The present invention relates to an ultrasonic apparatus and method which are especially adapted for material sealing or splicing as by joining together parts of relatively large size.
2. The Prior Art
The splicing or sealing of parts or sheets of material such as photographic film, resin coated photographic paper, and other types of thermoplastic resin films, such as, wrapping materials, has been practiced for some time. In such a procedure the sheets to be joined are positioned with their edges in overlapping relationship and an energized ultrasonic transducer horn is moved toward a cooperating anvil to seal the sheets or relative movement is effected between the horn and the sheets. The sealing energy is proportional to the product of the input electrical power supplied the transducer horn, the loading force or pressure applied to the transducer, and the frequency of vibration of the transducer horn.
Other ultrasonic operations are a field bonding of plastic parts as in U.S. Pat. No. 3,224,916; riveting of plastic parts as in U.S. Pat. No. 3,367,809; and sinking metal parts into plastic as in U.S. Pat. No. 3,184,353. Operations can be conducted with relative movement between the work and the ultrasonic horn as in U.S. Pat. No. 3,585,096, or without such motion.
In ultrasonic operations, and especially ultrasonic welding of plastic parts, it is important to hold the parameters of sealing time, input electrical power, and loading force close to optimum values to achieve consistently good results. Most commercially available ultrasonic equipment provide for adjustment of these parameters and further provide automatic frequency control (AFC) and/or automatic amplitude control. Such automatic controls are intended to maintain an "in tune" condition in spite of variations in the acoustic load during the sealing cycle and to prevent "run away" damage to the equipment in the absence of a load.
In high power applications of ultrasonics, it is essential and basic that (1) the source and load impedances be properly matched, and the (2) the system operate at the natural resonant frequency of the transducer and horn combination. In applications requiring intermittent use of ultrasonic power, such as welding of plastic assemblies, it is further important that power turn-on characteristics be repeatable and as rapid as possible so that each seal is permitted to receive the full amount of applied energy. Also, during the period of each weld the changing character of the load causes the reflected load impedance and mechanical resonant frequency to vary. It is often desirable to limit the power to the transducer when it is not loaded to prevent excessively high amplitude vibrations and heat build up, either of which could cause permanent damage to the transducer/horn assembly.
Some of these factors have been recognized for some time and numerous patents have been issued. For example, U.S. Pat. No. 2,917,691, provides means for maintaining operation at the transducer resonant frequency for purpose of maximum power transfer under conditions of varying load; however, no means are provided to maintain a prescribed power level or to limit power to a safe level with no load present.
U.S. Pat. No. 3,447,051, Atwood et al., provides automatic frequency control (AFC) and also senses transducer current, the amplifier of which is related to the amplitude of transducer vibration. The amplitude of transducer current is held constant by means of feedback and a variable gain circuit. This automatic amplitude control constitutes automatic power control (APC). When the intended operation is completed, a manually operated switch is provided which, when operated, reduces the transducer current to a safe value. The circuits are not shut down completely, thus minimizing any delay before operation is resumed. A problem is some times encountered with these circuits when it is desired to cool the sealed material before reducing the loading force on the transducer. Since there is no provision for terminating the power supply to the transducer, the transducer is always vibrating. This vibration can have an adverse effect on the quality of the seal and can also increase the amount of time required to cool the sealed material.
In U.S. Pat. No. 3,443,130, motional current supplied to a transducer is limited to a substantially constant value, which also protects the transducer in the absence of its intended load. A rather large impedance, preferably reactive, in a series circuit with the transducer is used with operation of the transducer at a frequency different from its natural mechanical resonant frequency. This system fails to make most efficient use of the system components. Further, these methods are not suitable for high power operation with a large horn. If operation can be achieved, the power obtainable is less than the rated value for the equipment. Also, turn-on is erratic in that the system does not always repeat the same operating frequency and power level.
In summary, when sealing parts of relatively large size such as film cassettes, and where the lateral dimensions of the ultrasonic horn of the transducer approach or even exceed an acoustic half wavelength, the increased power requirements make it desirable to maintain tuning to develop maximum power. It is also difficult to design a horn free of spurious resonances near the desired operation frequency. The horn can be started vibrating on the correct frequency when unloaded; therefore, it is common practice to apply power before the horn contacts the workpiece by sensing the proximity of the horn to the workpiece, or by sensing the applied force of the horn against the work before full force is developed, and then switching on power. In either case, full power is switched on immediately. Even when these techniques are used, occasional poor seals are produced.
Thus, in applying power to a transducer, particularly where a large horn is used, it is difficult to assure that the apparatus will operate at the desired frequency since such large horns often have harmonics near the desired operating frequency. If the desired frequency is not achieved, the resulting operations may be faulty, or at least of an uncertain quality level. While initiating operation at the desired frequency is more certain when the transducer horn assembly is started under little or no load, this conflicts with the fact that the transducer horn assembly can be damaged if operated under no-load conditions. Also, bringing an already vibrating horn into contact with a workpiece can mar the surface at the point of contact, resulting in cosmetic defects in the product. The present invention resolves the conflict among the foregoing factors by initially applying a low level of power to the transducer horn assembly before it is placed under load thereby more readily initiating operation at the desired frequency. This power level is at a sufficiently low level that the risk of damaging the assembly and/or the workpiece is avoided. As the transducer horn assembly is loaded by contact with the workpiece, the power is increased to a level sufficient to provide the desired operations on the workpiece with the requisite quality.